1. Field of the Invention
The present invention relates to a magnet unit which is disposed on the backside of a cathode electrode supporting a target on the front side in sputtering and relates to a magnetron sputtering apparatus provided with the magnet unit.
2. Description of the Related Art
A magnetron sputtering apparatus generates magnetron on a target discharge surface using a magnet unit which is disposed on the back side of a cathode electrode supporting the target and confines the plasma to obtain a high density state thereof. Then, when an ion of the plasma generated in this apparatus collides with the target, target material is flicked out and attached to a substrate to form a thin film. The sputtering used in a deposition process of the semiconductor industry deposits a film from any kind of source material, and can deposit a high melting-point material such as platinum and tungsten and an insulating material, for example. In addition, the above sputtering is a method in which sputter particle energy is easily changed and it is also possible to control the crystalline state, magnetic property, insulating characteristic, stress, and the like of a film.
In a sputter phenomenon used in the sputtering process, glow discharge is generated on the target front, and an ion is extracted from the plasma generated by this glow discharge toward the target and accelerated to collide with the target surface, and thereby the ion flicks a sputter particle from the target surface as material of a film.
Then, a sputter cathode used in the sputtering process utilizes the following principle. A cathode magnet is disposed in the air on the back side of the target which is disposed in vacuum and the both are separated by a separation wall (e.g., back plate). Then, a magnetic tunnel is formed on the target surface having an endless track shape by a magnetic force line generated from a cathode magnet (here, in the magnetic tunnel, a group of points where a component perpendicular to the target surface is zero is called a “magnetic track”). When electric power is applied to the target in this state, an electric field is generated in the normal line direction of the target surface. An electron is confined in a region where the magnetic field and the electric field cross perpendicularly to each other, and the confined electron collides frequently with gas element to turn the gas element into an ion. The ion is accelerated by an electric field generated on the target front and causes the sputtering.
Meanwhile, the ion flicks an element on the target surface and thereby the target surface is eroded along with the elapse of operation time. When this erosion depth becomes close to the thickness of the target, the target needs to be replaced by a new one. Erosion speed and shape change depending on various factors such as an electric field magnitude and a magnetic flux density generated on the target front, a gas pressure during the sputtering, and the shape of the magnetic track. When the erosion speed is higher, the target needs to be replaced more frequently and the operation rate of the sputtering apparatus is reduced. On the other hand, the erosion speed is lower, the target needs to be replaced less frequently, and the operation rate of the sputtering apparatus can be increased. In many cases, this erosion progresses selectively in a partial region of the target surface (or, in a circumferential part thereof) and causes the erosion speed to become higher. It has been known that a factor greatly affecting the erosion speed and shape is the magnetic track, and many proposals have been provided regarding a cathode magnet shape for generating the magnetic track.
FIG. 23 is a plan view showing an arrangement pattern of permanent magnets disclosed by Japanese Patent Application Laid-Open Publication No. 2009-7637. The technique shown in FIG. 23 and described in Japanese Patent Application Laid-Open Publication No. 2009-7637 is a proposal regarding the cathode magnet shape. In FIG. 23, plural magnets 20 are disposed forming a magnetic field for confining the plasma, and the magnets 20 include magnets 20A and magnets 20B for plural magnet pairs. Further, the plural magnets 20A and 20B are configured to be rotatable by means of a rotation mechanism (not shown in the drawing). The plural magnets 20A and 20B are arranged so as to form a magnetic field extending to cross over a closed curve in the vicinity of the target surface and the rotation center exists in a region surrounded by the closed curve. The closed curve has plural convex parts and plural concave parts, and the distances from the convex parts to the rotation center are configured to be different from one another and also the distances from the concave parts to the rotation center are configured to be different from one another.
In Japanese Patent Application Laid-Open Publication No. 2009-7637, a magnetic track curve is configured to be longer at a position farther from the rotation center and the distances to the tops of the convex parts and to the concave parts are configured to be different from one another, in order to make the erosion speed the same at each radius. However, it is difficult to project an erosion point and the erosion speed at the time of magnetic circuit design performed only by way of such a geometric shape. In particular, this is because the plasma density on the target front which determines the erosion speed depends also on the magnetic flux density. For example, if the magnetic flux density is changed at a position even on the magnetic track having the same length, the erosion speed changes.
This point will be explained by the use of FIG. 25. FIG. 25 is a diagram showing a curve corresponding to a horizontal magnetic field position formed by the arrangement pattern of the magnets in FIG. 23. Here, a position of a part where the magnetic force line becomes parallel to the surface of the target is called a “horizontal magnetic field position”. In Japanese Patent Application Laid-Open Publication No. 2009-7637, when the tops of convex parts A, B, and C (farthest points from the rotation center P) shown in FIG. 25 are denoted by a, b, and c, respectively, the distance from a rotation center P to the top a of the convex part A, the distance from the rotation center P to the top b of the convex part B, and the distance from the rotation center P to the top c of the convex part C are different from one another. Similarly, when the bottoms (positions closest to the rotation center P) of convex parts D, E, and F are denoted by d, e, and f, respectively, the distance from the rotation center P to the bottom d of the concave part D, the distance from the rotation center P to the bottom e of the concave part E, and the distance from the rotation center P to the bottom f of the concave part F are different from one another.
The method of causing the erosion to progress uniformly as explained in Japanese Patent Application Laid-Open Publication No. 2009-7637 assumes that the magnetic flux density is the same among the above convex parts A, B, and C and concave parts D, E, and F. It has been confirmed that the magnetic flux density are not always uniform on the magnetic track as shown in FIG. 26, when the magnetic flux density is obtained by simulation on the magnetic track of the target surface. This is because a density in the number of the magnets changes at a curved part when the magnets are disposed in a curve.
As described above, Japanese Patent Application Laid-Open Publication No. 2009-7637 assumes that the erosion progresses uniformly across the whole target by way of making an arc length of the magnetic track longer at a farther position from the rotation center, considering the movement speed of the magnetic track is higher at a position farther from the rotation center. However, Japanese Patent Application Laid-Open Publication No. 2009-7637 does not explain to the point whether the magnetic flux density is uniform or not on the magnetic track, and the erosion does not always progress uniformly there.
In particular, when the target is made of magnetic material, most of the magnetic force lines from the cathode magnet pass through the inside of the target, and thereby it is difficult to generate the magnetic track on a magnetic circuit in the same manner as in the design shown in FIG. 24A. Accordingly, sometimes the erosion speed becomes higher in an unexpected position and a target life becomes shorter. FIG. 24A shows a result of simulation of the magnetic track on the target surface when a magnetic target (saturated magnetic flux density: 1.0 T, thickness: 3 mm, and diameter: 160 mm) is applied to the magnetic circuit disclosed in Japanese Patent Application Laid-Open Publication No. 2009-7637. In FIG. 24A, Reference numeral 21 is a designed curve of a curve corresponding to the horizontal magnetic field position formed by the magnet arrangement pattern shown in FIG. 23. Further, Reference numeral 22 is an eroded region in the case of using the magnetic target. From this result, particularly in a case such as one in which the target is made of magnetic material, there are many regions in which the magnetic track does not overlap the magnetic circuit in an area on the left side of the magnet center in FIG. 24A. Further, a difference of the radii between from the target center to the convex parts and from the target center to the concave parts of the magnetic track has almost disappeared in the left area of this target.
Further, in the magnetic target, since the magnetic field is concentrated more at an eroded position as the erosion progresses, the magnetic field concentration occurs at the eroded position even when the most-deeply eroded part and the magnetic circuit does not coincide with each other as shown in FIG. 24B. That is, there is a limitation for a method in which the magnetic track includes many regions parallel to the rotation direction of the magnet unit and radii are shifted from one another in the convex and concave parts of the magnetic tracks, as the method of Japanese Patent Application Laid-Open Publication No. 2009-7637. In particular, when the target diameter is increased, it is difficult to cause the erosion to progress uniformly across the whole surface of the target without providing many radii for the convex parts and concave parts of the magnetic track. Note that, in FIG. 24, Reference numeral 23 indicates a magnetization direction in a minute region within the target and Reference numeral 24 indicates a magnetic force line on the target surface.